Abrasive grains with the most different grain sizes, in bound and loose form, are used for the most diverse grinding processes, with which all of the known materials can be processed. The use of abrasive grains in bound form differentiates between the so-called bonded abrasives, which are understood to be grinding wheels, abrasive stones or also mounted wheels, with which the abrasive grains are formed into the corresponding abrasive by means of a ceramic mass or by means of a synthetic resin and are subsequently solidified by means of a heat treatment as well as the coated or flexible abrasives, respectively, with which the abrasive grains are fixed on a support (paper or textile) by means of a binder (synthetic resin).
The efficiency of the different abrasives does not only depend on the abrasive grain, which is used, but it also largely depends on the integration of the abrasive grain in the abrasive. Particular importance is thereby accorded to the interface between abrasive grain and binder phase, because said interface determines the force, which is necessary for breaking an abrasive grain out of a bond. The harder and more ductile an abrasive grain, the higher the demands on the bond and on the adhesive forces at the interfaces. Most of the abrasive grains, in particular those produced by means of a melting process, have a relatively smooth surface, which has proven to be disadvantageous for the integration. Grinding operations where more than 50% of the abrasive grain is lost by breaking out of the bond are thus not a rarity and are thus not used at all for the actual grinding application.
In the past, a plurality of methods for respectively roughening and increasing the surface of the abrasive grain and thus for improving its integration have been proposed and taken. Most of these methods are based on applying micro-particle pigments or powder onto the surface of the abrasive grain and to firmly bond them with the abrasive grain. For this purpose, the surface of the abrasive grain is generally wetted with a binder and is subsequently mixed with an inorganic pigment or powder so that, if possible, the grain surface is evenly and homogenously coated with a layer of micro particles. Subsequently, the abrasive grains, which are treated in such a manner, are subjected to a heat treatment, in response to which the bond between fine grain particles and abrasive grain is solidified.
Silica-based binders, such as, e.g., sodium silicate or colloidal silicon dioxide are frequently used as binders. A disadvantage of this treatment is that relatively high temperatures are required for achieving a solidification of the binder so that not only a relatively high amount of energy is used for such treatments, but so that temperature-sensitive abrasive grains are also excluded from a coating of this type.
U.S. Pat. No. 2,527,044 A describes a fused aluminum oxide or silicon carbide abrasive grain, which is coated with a coating consisting of fine-grain metal oxide particles, for example iron(3)oxide or molybdenum oxide for the purpose of improving the integration, in particular in synthetic resin-bound abrasives. In this case, a low-melting glass frit is used as a binder and the temperature of the oxidizing heat treatment lies between 1350° and 1500° F., which corresponds to a temperature of 732° C. to 1222° C. Such a treatment is not suitable for temperature-sensitive and/or oxidation-sensitive abrasive grains, such as, e.g., eutectic zirconium aluminum oxide, cubic boron nitride or diamond.
EP 0 014 236 A1 describes the treatment of abrasive grain on the basis of aluminum oxide, wherein a layer consisting of a ceramic mass, such as, e.g., clays, kaolin or glass frits, is melted or sintered onto the abrasive grain. At the same time, a conversion of the titanium oxide included in the aluminum oxide is to take place from the trivalent to the tetravalent oxidation stage by means of the sintering or melting of the coating. This coating method is thus exclusively suitable for abrasive grains, which encompass portions of titanium oxide in addition to aluminum oxide. In addition, the heat treatment is to occur at a temperature of from 1250° C. to 1350° C. and is to take place under oxidizing conditions so that this treatment can also not be considered for oxidation and temperature-sensitive abrasive grains.
Low temperatures for the heat treatment can be realized by means of phosphate binders or organic binders, such as, e.g., synthetic resin. However, these binders have the disadvantage that they have a relatively low adhesive power to the abrasive grain and the stability of the coating on the surface is thus inadequate.
DE 102 57 554 A1 describes abrasive grains from the group of the conventional abrasive grains, in particular melted or sintered aluminum oxides, zirconium aluminum oxides, silicon carbide and boron carbide for use in synthetic resin-bound abrasives, the surface of which is provided with a coating consisting of an aqueous binder on the basis of silicate and a fine-grain oxide compound. The fine-grain oxide compound is a complex compound of the general formula AxByOz with one element A from the group of metals and one element B from the group of amphoteric elements as well as oxygen in the stoichiometric ratio to A and B.
For the heat treatment, provision is made for a temperature range between 100° C. and 900° C. and, after a heat treatment of the coated abrasive grain at 400° C., considerably increased performances can be achieved in comparison to an untreated abrasive grain. More accurate tests of abrasive grains, which are treated in such a manner, in particular the determination of the bond strength by means of acoustic cavitation, have shown that the bond of the oxide compound at the surface is relatively weak and that these increased performances have been achieved even though the bond of the coating at the abrasive grain surface is weak and the increased performances can possibly be ascribed to the crystal structure of the pigment (rutile lattice) and the composition of the pigment comprising an amphoteric element, which possibly supports the grinding process as auxiliary abrasive.
Furthermore, the problem thus exists of finding a binder system for a coating of abrasive grains, which does not encompass the disadvantages of the state of the art and which forms a firm bond with the abrasive grain surface already at low temperatures. With such a binder system, it should be possible to achieve further improvements of the grinding performances, in particular for thermally unstable abrasive grains. In particular the treatment of eutectic zirconium aluminum oxide, the pyrolysis of which starts at temperatures of above 400° C. caused by modification conversions of the zirconium oxide and volume changes connected therewith, lies in the center of interest.